Researchers also found that males will expend extra energy on their mating callincreasing the tempo and prolonging the duration of each callto take advantage of the favorable acoustics.

"As far as we are aware, [this] is the first evidence that an animal can actively alter its behavior, its call pitch, to obtain resonance [to] produce this super-attractive call, which is quite complex," said Björn Lardner, one of two researchers to make the discovery.

Lardner, an animal ecologist at the University of Lund, Sweden, and his research partner, Maklarin bin Lakim, of the Sabah Parks Research and Education Division in Sabah, Malaysia, conducted six months of field research in Borneo's Kinabalu National Park. Lardner later analyzed audio field recordings and other data at the Field Museum of Natural History in Chicago, Illinois.

The discovery marks the first time an animal species has been shown to alter its calling behavior to exploit resonance effects. Some species of burrowing frogs and crickets are known to dig burrows or cut baffles in leaves to specific dimensions to create a resonant effect for their mating calls. But such adaptations involve the insect or animal changing their physical environment, not altering their individual calling behavior.

Since such behavior was not previously known to science, researchers have rarely looked for such traits in animals, Lardner suggested. The findings raise the possibility that similar overlooked animal behaviors await discovery, he said.

"The question that arises is, how common is this phenomenon?" said Lardner. "Is it that we haven't looked for it in animals? Could it be that animals are using [other] signals in more complex ways of exploiting the physics of the environment in which they live?"

An Unexpected Discovery

Borneo tree-hole frogs grow up to one inch (2.5 centimeters) in size and are endemic to the lowland rain forests of Borneo. They mate and breed in tree cavities formed by rotting broken limbs. Frequent rainfall can partially fill these cavities with water. Female tree-hole frogs lay their eggs in these miniature, sheltered standing ponds after mating.

To study the frog's mating-call strategies, researchers Lardner and bin Lakim recorded more than 300 mating call events by frogs in their natural habitat.